Patent GB-680866-A (GB)
Title: Improvements in and relating to a sanding machine
IPC: B24B23/04
Filed: 1950-08-05
Abstract: 680,866. Portable sanding-machines. DOBSON, F. A. Aug. 5, 1950, No. 19635/50. Class 60. [Also in Group XXVI] A sanding-machine comprising a frame and a pad assembly suspended in the frame and arranged to oscillate in the longitudinal direction of the frame, is characterized by means for balancing the inertia force applied to the frame by the oscillating pad assembly, comprising means for applying an inertia force in one direction and means for applying another inertia force in another direction, the means being so arranged that the paths of the inertia forces: applied by these means will intersect at a point substantially in the path of the inertia force of the pad assembly and so that the resultant of the inertia forces applied by the means is substantially equal and opposite in direction to the: inertia force of the pad assembly. An electric motor 10 is mounted in a frame 12 provided with a handle 18 having a switch 20 for controlling the motor; a removable handle 22 is also provided. The motor shaft 14 is journalled in bearing 16 in the frame and carries at its lower end an eccentric 24 operating a connecting rod 26, a ball-bearing 28 being provided; the other end of the connecting rod 26 is pivotally secured to a carriage 30 by a. rubber bushing 32. To protect the eccentric: bearing from misalignment the connecting rod comprises two sections 34, 36 joined by a flexible leaf spring 38; alternatively misalignment may be prevented by using a self-aligning bearing, a bearing mounted in rubber or a connecting rod of flexible material. To eliminate vibration due to rotation of eccentric 24, an off-set counter weight is secured to the motor shaft 14 and is so proportioned that it balances the inertia force of eccentric 24, bearing 28 and end 34 of connecting rod 26. The carriage 30 is a rectangular frame with side members 40, 42 which are pivotally connected to the frame 12 by rocker arms 44 providing a parallelogram linkage. Rubber bushings 46 are press fitted into the ends of side members 40, 42; a tubular spacer 48 extends between the bushings 46 and the rocker arms 44 are clamped against the bushings by a nut and bolt 50. The rocker arms 44 are pivotally attached to the frame 12 by rubber bushings 54 press fitted into the frame at 56; the rocker arms are clamped between the bushings 54 and a tubular spacer 58 by nut and bolt 60. Movement of the rocker arms 44 takes place by means of shear in the rubber bushings 46, 54. The carriage 30 is secured to a sander plate 62 by means of two pairs of flexible leaf springs 64 extending across the side members 40, 42 and secured thereto. The leaf springs 64 in eachpair are secured together at their mid-point by a stud 66 and a spacer 68 passing through the bushings 32. The lower end of stud 66 has a ball-shaped head 70 adapted to seat in a cooperating slotted socket 72 formed in plate 62; the head 70 is removably retained in socket 72 by a spring 74 acting between the peened lower end 75 of the stud and a retaining washer 76 abutting against the lower face of plate 62; the slotted sockets 72 are provided with enlarged openings at one end thereof which permit the passage therethrough of retaining washers 76, to enable the sander plate to be removed from the carriage. A resilient pad 80 is secured to the bottom face of plate 62 and abrasive material 82 is cemented to pad 80 by a tacky adhesive which permits removal of the material for replacement. Resilient pads 84 may be interposed between the carriage 30 and plate 62. Alternatively, auxiliary rocker arms 102 may be secured to rocker arms 44 their lower ends bearing upon pads 104 secured to plate 62; the bearing surface 106 of each arm 102 is cylindrical, the axis of the cylinder coinciding with the axis of the bushing 54. The upper ends of the rocker arms 44 are inclined inwardly towards the centre of the machine and support cylindrical balance weights 86, the angular inclination of the upper portion of the arms 44 being such that the inertia forces resulting from the acceleration of balance weights 86 intersect at approximately the centre of gravity of the pad and carriage assembly; the resultant of these inertia forces is a horizontal force acting in a direction opposite to the direction of the inertia force resulting from the acceleration of the pad and carriage assembly, this resultant passing through the centre of gravity of the pad and carriage assembly. The masses of weights 86 are such that the resultant of their inertia forces is exactly equal to the inertia force of the pad and carriage assembly. In a modification, Fig. 6, the rocker arms 108 are integral with transverse springs 110 which deflect torsionally with the pivotal movement of the rocker arms. Rigid spacers 112 secured to the mid-points of the springs 110 are provided with threaded studs 114 for attaching the pad plate 116 to rocker arms 108. At the lower end of rocker arms 108, where they bend to form transverse springs 110, the bend is made so that the lower surface of springs 110 is cylindrical, its axis coinciding with the pivoted axis of the rocker arms; these cylindrical surfaces are supported by resilient pads 120 on the plate 116. To form the cylindrical surface on springs 110 without unduly stretching the metal at the bend, the adjacent vertical portion of the rocker arm is formed to an equal and opposite curvature; this reinforces the rocker arm and resists lateral flexing. Tension springs 122 extend between the rocker arm bolts 124 and blocks 126 fixed to springs 110. The connecting rod comprises a vertically disposed leaf spring 128 clamped at one end to one of the blocks 126 and having a yoke 130 secured to its other end; the other end of yoke 130 is attached to the ends of a horizontally disposed leaf spring 132 carried by the bearing collar 134. The motor may drive the pad through speed reducing gearing.